Stabilizing feedbacks in glacier-bed erosion

Abstract

Glaciers often erode, transport and deposit sediment much more rapidly than nonglacial environments, with implications for the evolution of glaciated mountain belts and their associated sedimentary basins. But modelling such glacial processes is difficult, partly because stabilizing feedbacks similar to those operating in rivers have not been identified for glacial landscapes. Here we combine new and existing data of glacier morphology and the processes governing glacier evolution from diverse settings to reveal such stabilizing feedbacks. We find that the long profiles of beds of highly erosive glaciers tend towards steady-state angles opposed to and slightly more than 50 per cent steeper than the overlying ice-air surface slopes, and that additional subglacial deepening must be enabled by non-glacial processes. Climatic or glaciological perturbations of the ice-air surface slope can have large transient effects on glaciofluvial sediment flux and apparent glacial erosion rate.

Ice flow models and glacial erosion over multiple glacial–interglacial cycles

“…For these model comparisons, no below-sea-level overdeepenings develop. While this is a simplification of the complex feedbacks among the glacial flow, subglacial and englacial hydrology, and the subglacial sediment (Hooke, 1991;Alley et al, 2003), glaciated valleys are produced that fit the expectations that overdeepened glaciers tend to reach critical angles. For the model comparisons, all processes except for glacier sliding and deformation are calculated within the ICE-Cascade model not within the HO nested model.…”

“…We evaluate not just how the glacial-flow physics model influences the erosion of topography but also how this topography and the eroded sediment work within the other geomorphological systems. Fluvial erosion is an important mechanism, and the fluvial network is also responsible for the transport and redistribution of glacially eroded sediment once it has left the toe of the glacier Alley et al, 2003). In some cases, the existing rivers do not have the carrying capacity to support the evacuation of all the available eroded material.…”

Relative size of fluvial and glaciated valleys in central Idaho

“…Cirque headwalls and overdeepenings are genetically similar and result primarily by plucking of bedrock rather than abrasion (Hooke, 1991;Oskin and Burbank, 2005). The plucking of bedrock in overdeepenings is facilitated, in part, by the addition of meltwater via fractures in the ice, which contributes to variability in glacier velocity patterns and subsequent freeze-thaw dynamics related to pressure fluctuations in basal water pressure (Hooke, 1991;Alley et al, 1999Alley et al, , 2003. Blocks at the bed are loosened by pressure-release fracturing and crack propagation facilitated by the tremendous pressure differentials above the bedrock over small spatial scales, which result from freeze-thaw dynamics as the ice interacts with asperities on the bed (Hooke, 1991;Iverson, 1991).…”

“…In contrast, in polar latitudes, or during periods of cold climate, freezing of glaciers to their beds is possible. In both cases, local variation in temperature profiles at the glacier bed are driven by seasonal subglacial water fluxes and heat flow feedbacks between the ice surface slope and the ice bed, particularly in areas of overdeepening (Alley et al, 1999(Alley et al, , 2003. Hallet (1979Hallet ( , 1996 proposed mechanical models for glacial abrasion and quarrying (plucking) that apply to glaciers whose basal ice has a relatively sparse load of rock fragments.…”

How dynamic are ice-stream beds?

“…For example, previous studies have theorised that high subglacial till fluxes can lead to the rapid formation of grounding-zone wedges, potentially stabilising ice streams against sea-level rise (e.g. Alley et al, 1989Alley et al, , 2003Alley et al, 2007).…”